Semiconductor nanocrystals whose dimensions are comparable to the bulk exciton diameter show quantum confinement effects. This is seen most clearly in the optical emission spectra which shift towards the red and/or infrared as the size of the crystal is increased.
Semiconductor nanocrystals made from a wide range of materials have been studied including many II-VI and III-V semiconductors. In addition to spherical nanocrystals rod-, arrow-, teardrop- and tetrapod-shaped nanocrystals [Alivisatos et. al., J. Am. Chem. Soc, 2000, 122, 12700; WO03054953] and core/shell structures [Bawendi, J. Phys. Chem. B, 1997, 1010, 9463; Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] have also been prepared. To control the size and shape of such nanocrystals their synthesis is generally performed in the presence of one or more capping agents (sometimes called surfactants or coordinating solvents). Such capping agents control the growth of the nanocrystals and also increase the intensity of the light emission though the passivation of surface states. A wide range of capping agents have been employed including phosphines [Bawendi et. al., J. Am. Chem. Soc., 1993, 115, 8706], phosphine oxides [Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], amines [Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], fatty acids [Battaglia and Peng, Nano Lett., 2002, 2, 1027; Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], thiols [Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] and more exotic capping agents such a metal fatty acid complexes [Nann et. al., J. Mater. Chem., 2008, 18, 2653].
Methods to prepare semiconductor nanocrystals include solvothermal reactions [Gillan et. al., J. Mater. Chem., 2006, 38, 3774], hot injection methods [Battaglia and Peng, Nano Lett., 2002, 2, 1027], simple heating processes [Van Patten et. al., Chem. Mater., 2006, 18, 3915], continuous flow reactions [US2006087048] and microwave assisted synthesis [Strouse et. al., J. Am. Chem. Soc., 2005, 127, 15791].
Nitride nanocrystals have been produced before by Sharp [UK Patent applications GB2467161, GB2482311, and GB2467162]. However, the synthesis method included a long ligand and required temperatures in excess of 150° C.
WO 2008/094292 proposes the growth of a nitride nanocrystal shell, for a nanocrystal having a core of a group II alloyed I-III-VI material, by the pyrolysis (that is, thermal decomposition) of organometallic precursors in a chelating ligand solution.
JP 2004/307679 proposes the gaseous phase growth of indium nitride nanoparticles using triethylindium and nitrogen as precursors.
US 2008/160306 proposes a nanostructure growth method in which a molecular cluster compound is used to seed the growth of nanoparticles from a reaction mixture.
WO 2010/118279 proposes the growth of a metal sulphide shell around a nanocrystal core using zinc acetate as the metal precursor in formation of a zinc sulphide shell.
GB 2429838 and GB 2472541 propose methods for the synthesis of nanocrystals of ZnS, ZnSe, CdS etc. The use of organometallic precursors is mentioned, although both documents investigate other materials in view of the handling difficulties described as associated with organometallic compounds.